At a debate in which the two sparred almost cordially, Obama suggested Clinton had taken a page from President Bush's political playbook with an earlier statement that the next president could expect to be tested quickly by terrorists.
"When Senator Clinton uses the specter of a terrorist attack with a new prime minister during a campaign, I think that is part and parcel with what we've seen, the use of the fear of terrorism in scoring political points, and I think that's a mistake," he said.
Asked by NBC's Tim Russert whether she had meant to say terrorists would test Obama more than her, she replied, "No, of course not," before adding, "it matters who's president."
Clinton, Obama and former Sen. John Edwards, the only white man among the candidates on stage, settled in for their debate as the former first lady won a meaningless Michigan presidential primary, a contest held in violation of party rules.
The debate also unfolded four days before the party-sanctioned Nevada caucuses, the next for-keeps contest in the wide-open race for the party's presidential nomination.
Race dominated the debate at the outset, after several days of back-and-forth that left many Democrats worried about an adverse impact on the party's prospects for the general election.
Obama said "not only in hindsight, but going forward," he regretted that his staff had prodded reporters to pursue the issue.
"Our supporters, our staff, get overzealous. They start saying things that I would not say," added the most viable black candidate in history.
"We both have exuberant and sometimes uncontrollable supporters," Clinton said in the opening moments of a two-hour, round-table debate televised on MSNBC. "We need to get this campaign where it should be," said the former first lady, seeking to become the first woman to occupy the White House.
She said comments by black businessman Robert Johnson over the weekend were inappropriate, but sidestepped when asked whether she would bar him from playing a role in her campaign. Johnson made an evident reference to Obama's youthful drug use - although he denied that was his intent.
Obama won the kickoff Iowa caucuses less than two weeks ago, and Clinton countered with an upset victory last Tuesday in the New Hampshire primary. Edwards is winless. After Nevada, the South Carolina Democratic primary is Jan. 26, then the campaign explodes with nearly two dozen contests on Feb. 5.
There was a political reason for the pleasantries, underscored when Edwards was asked whether he and Obama had teamed up to attack Clinton in a debate just before the New Hampshire primary.
"I don't think it was that way," he said. "My job as a candidate for president is to speak the truth as I see it."
Clinton won the primary in an upset three days after the debate, carried to victory over Obama by an unexpectedly large turnout by women voters.
At the same time, there were limits to the comraderie, and Clinton, in particular, took several opportunities to challenge her rivals.
Asked whether Edwards and Obama were prepared to sit in the White House, she said "that's what the voters have to decide."
Later, Clinton asked Obama to back her legislation to prevent Bush from unilaterally extending the United States' presence in Iraq beyond the end of his term next January.
"I think we can work on this, Hillary," he replied.
On an issue of particular interest in Nevada, the former first lady stressed her opposition to a plan to develop a nuclear waste repository at Yucca Mountain in the state. She said Obama's campaign drew financial support from a company that favors the project, and in a rare campaign jab at Edwards, said he had twice voted for it.
Obama countered, saying that even though his home state of Illinois has more nuclear power plants than any other, his opposition to the controversial project was unequivocal.
Edwards said he had changed his mind on the issue, citing new scientific evidence about its risks.
Edwards, given a chance to question his rivals, pointed out the huge sums that Clinton and Obama have raised from drug and insurance companies. "Do you think these people expect something or are they just interested in good government?" he asked.
Obama quickly replied that he did not accept donations from federal lobbyists or political action committees, and Edwards just as quickly pointed out that applied to him as well.
All three also agreed they would not seek creation of a national gun registry, a shift in position for Clinton.
The Michigan primary was an election in name only, where Clinton was the only major candidate entered. She faced competition principally from the "uncommitted" line on the ballot, an option that some supporters of Edwards and Obama advocated to embarrass her.
Returns from 87 percent of the state's precincts showed her with 56 percent of the vote, and uncommitted gaining about 39 percent.
Pre-caucus polls in Nevada make it a close race among the three, an event spiced by a lawsuit filed by several Clinton supporters hoping to challenge the ground rules.
Their objective was to prevent several caucuses along the Las Vegas Strip, where thousands of Culinary Workers Union employees - many of them Hispanic or black - hold jobs.
The rules were approved in May, when Clinton was the overwhelming national front-runner in the race. But the union voted to endorse Obama last week, and the lawsuit followed.
Ohio Rep. Dennis Kucinich had hoped to have a seat at the table, but the Nevada Supreme Court ruled shortly before the debate began that MSNBC was legally entitled to prevent him from participating. It promptly did.
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Associated Press writer David Espo reported from Washington.
